1. Field of the Invention
The present invention relates to a method of air/fuel ratio control for an internal combustion engine.
2. Description of Background Information
In order to reduce the level of exhaust gas pollutants and improve the fuel consumption of an internal combustion engine, it is known to employ an oxygen concentration sensor for the detection of the concentration of oxygen in the engine exhaust gas, and to use feedback control to maintain the air/fuel mixture supplied to the engine at a targeted air/fuel ratio. This feedback control is performed using with an output signal from the oxygen concentration sensor.
One type of oxygen concentration sensor which can be employed for such air/fuel ratio control produces an output which varies in proportion to the oxygen concentration in the engine exhaust gas. This oxygen concentration sensor is disclosed in Japanese patent laid-open No. 52-72286. This oxygen concentration sensor consists of one oxygen ion-conductive solid electrolytic member formed as a flat plate having a pair of electrodes respectively formed on its two main faces, with one of these electrode faces forming part of a gas holding chamber. The gas holding chamber communicates with the exhaust gas to be measured, through a lead-in aperture. The oxygen ion-conductive solid electrolytic member and its pair of electrodes function as an oxygen pump element. By passing current between the electrodes, the electrode within the gas holding chamber will become a negative electrode, and the oxygen gas within the gas holding chamber adjacent to this negative electrode will become ionized and flow through the solid electrolytic member towards the positive electrode. The ionized oxgyen is then emitted from the face of the pump element as gaseous oxygen. The current flow between the electrodes is lower than a boundary current value which is substantially constant. Thus it is substantially unaffected by variations in the applied voltage and is proportional to the oxygen concentration within the gas being measured. Therefore, by sensing the level of this boundary current the oxygen concentration of the exhaust gas can be measured. However, if the oxygen concentration sensor is used to control the air/fuel ratio of the mixture supplied to an internal combustion engine, by measuring the oxygen concentration within the engine exhaust gas, the air/fuel ratio can only be maintained at a value in the lean region relative to the stoichiometric air/fuel ratio. It is not possible to maintain the air/fuel ratio control to at targeted air/fuel ratio in the rich region. An oxygen concentration sensor which will provide an output signal level varying substantially in proportion to the oxygen concentration in engine exhaust gas for both the lean region and the rich region of the air/fuel ratio has been disclosed in Japanese patent laid-open No. 59-192995. This sensor consists of two oxygen ion-conductive solid electrolytic members each formed as a flat plate and provided with a pair of electrodes. Two opposing electrode faces, one face of each of the solid electrolytic members, form part of a gas holding chamber which communicates the gas to be measured via a lead-in aperture. The other electrode of one of the solid electrolytic members faces the atmosphere. One of the solid electrolytic members and its electrodes functions as an oxygen concentration ratio sensor cell element. The other solid electrolytic member and its electrodes functions as an oxygen pump element. If the voltage which is generated between the electrodes of the oxygen concentration ratio sensor cell element is higher than a reference voltage value, the current will flow between the electrodes of the oxygen pump element such that the oxygen ions flow through the oxygen pump element towards the electrode of the element within the gas holding chamber. If the voltage developed between the electrodes of the sensor cell element is lower than the reference voltage value, the current will flow between the electrodes of the oxygen pump element such that the oxygen ions flow through the element towards the oxygen pump element electrode located on the opposite side to the gas holding chamber. In this way, a value for current flow between the electrodes of the oxygen pump element can be measured. This value varies substantially in proportion to the oxygen concentration in the exhaust gas, both in the rich and the exhaust lean regions in the air/fuel ratio.
However when an oxygen concentration sensor producing a varied output in proportion to the oxygen concentration is employed, variations in the sensors detecting characteristics and deteriorations of the sensor will occur as time elapses. As a result, the accuracy of the correspondence between the basic value which is set in accordance with one or more engine parameters related to the engine load and the target air/fuel ratio will be reduced, so that errors will arise. One method which could be used to counteract this problem is to compute compensation values to compensate the errors in the basic value. In addition to storing the output from the oxygen concentration sensor, these compensation values can be stored as data in memory locations which are determined in accordance with the specific engine operating region at the time of the computation of the compensation value. When the computation of the output value is performed, the appropriate compensation value corresponding to the current operating condition of the engine would be obtained by searching the stored data, and the compensation value would be used to compensate the basic value. By using such a method, the compensation values are computed in accordance with the oxygen concentration sensor output. Thus, if the compensation was computed during a large change in the oxygen concentration of the exhaust gas, the accuracy of air/fuel ratio control can actually be reduced, and decreasing the effectiveness of the elimination of exhaust pollutants.